Temperature measuring device

ABSTRACT

A temperature measuring device includes an optical fiber; a metallic protective tube for covering the optical fiber to form a metal-covered optical fiber; and a radiation thermometer connected to the metal-covered optical fiber. A heat insulation coating may be added to cover the metallic protective tube.

This is a division of application Ser. No. 08/310,227 filed Sep. 21,1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a temperature measuring device and alevel measuring device using the temperature measuring device.

2. Description of the Related Arts

In a continuous casting process, for example, accurate measurement oftemperature and surface level of molten steel during casting isnecessary fop improving the product quality and the production yield.Conventional art for measuring the temperature of molten steel in atundish and a mold prepares a solidification chamber inside of a carbonsleeve to introduce the molten steel, and determines the temperature inthe solidification chamber by a contact thermometer of a consumableimmersion thermocouple or a thermocouple covered with a ceramicprotective tube. As for the method to determine the level of moltensteel, conventional art uses an eddy-current distance detector.

The above-described consumable immersion thermocouple degrades afteronly one measurement because it directly contacts the molten steel.Therefore, the probe to measure the temperature is detachable at the tipof the thermometer, and the tip probe is replaced in every measurement.Since such an expensive probe is discarded in every measurement,increase of the number of measurements is difficult.

In the case that a thermocouple is covered with a ceramic tube, thethermocouple does not contact directly with the molten steel.Consequently, a continuous measurement is possible. In this case,however, the durability of ceramic protective tube has a limitationbecause of heat shock and erosion caused by slag. As a result, anexpensive protective tube is durable only to 40 to 50 hrs., and repeateduse for a long time is impossible.

Regarding an eddy-current distance detector which is used to determinethe level of molten steel, it is useful for a level control owing to aprecise measurement under a steady state condition. Nevertheless, therange of measurement is as narrow as 200 mm or less, so it can not beused to determine the level during an automatic start-up. Consequently,an automatization of automatic start-up mode is difficult.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a temperature measuringdevice which accurately determines the temperature of molten metal at anelevated temperature and to provide a level measuring device whichdetermines a arbitrary level using the temperature measuring device.

The present invention provides a temperature measuring devicecomprising:

(a) an optical fiber;

(b) a metallic protective tube for covering the optical tube to form ametal-covered optical fiber, a tip of the metal-covered optical fiberbeing a temperature measuring element; and

(c) a radiation thermometer connected to the metal-covered opticalfiber.

Moreover, the present invention provides a level measuring devicecomprising:

(a) an optical fiber;

(b) a metallic protective tube for covering the optical tube to form ametal-covered optical fiber, a tip of the metal-covered optical fiberbeing a temperature measuring element;

(c) a radiation thermometer connected to the metal-covered opticalfiber;

(d) transfer means for sending the metal-covered optical fiber to asurface of a molten metal to be measured and retracting to wind themetal-covered optical fiber from the surface; and

(e) level determination means for determining a level of the surface ofthe molten metal based on a temperature change detected by the radiationthermometer and an amount of feed of the optical fiber through thetransfer means.

The present invention provides another temperature measuring devicecomprising:

(a) an optical fiber;

(b) a protective tube for covering the optical fiber;

(c) a heat insulation coating for covering the protective tube, theoptical fiber being covered with the protective tube and the heatinsulation coating to form a double-covered optical fiber, and a tip ofthe double-covered optical fiber being a temperature measuring element;

(d) the optical fiber having a temperature at which the optical fiber isconsumed higher than a temperature of a molten metal to be measured; and

(e) the protective tube and the heat insulation coating having a heatresistant temperature of lower than a temperature of the molten metal tobe measured.

The present invention provides still another temperature measuringdevice comprising:

(a) an optical fiber;

(b) a metallic protective tube for covering the optical fiber;

(c) a heat insulation coating for covering the protective tube, theoptical fiber being covered with the protective tube and the heatinsulation coating to form a double-covered optical fiber, and a tip ofthe double-covered optical fiber being a temperature measuring element;and

(d) said heat insulation coating containing particles as an additivehaving a melting point higher than a temperature of a molten metal to bemeasured.

The present invention provides yet another temperature measuring devicecomprising:

(a) an optical fiber;

(b) a metallic protective tube For covering the optical fiber; and

(c) a heat insulation coating for covering the metallic protective tube,the heat insulation coating comprising cellulose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the present invention;

FIG. 2 shows a cross section of a covered optical fiber which is coveredwith a metallic tube of the present invention;

FIG. 3 illustrates a scheme of the present invention applied to atemperature measurement of molten steel in a mold of a continuouscasting machine;

FIG. 4 is a graph showing a characteristic curve of optical fiber feedtime and temperature change;

FIG. 5 is a graph showing a characteristic curve of optical fiber feedtime and temperature change;

FIG. 6 illustrates another embodiment of the present invention;

FIG. 7 illustrates a scheme of the present invention applied to atemperature and level measurement of molten steel in a mold of acontinuous casting machine;

FIG. 8 is a graph showing a characteristic curve of molten steel leveland temperature change;

FIG. 9 illustrates a scheme of the present invention applied to atemperature and level measurement of molten steel in a tundish;

FIG. 10 is a front view of a covered optical fiber which is covered witha metallic tube in another embodiment of the present invention;

FIG. 11 illustrates another embodiment of the present invention;

FIG. 12 shows a cross section or a double-covered optical fiber of thepresent invention;

FIG. 13 shows a cross section of the double-covered optical fiber in ameasuring state;

FIG. 14 shows a cross section of the double-covered optical fiber undera burnt-damage state;

FIG. 15 shows an observed waveform;

FIG. 16 shows an observed waveform;

FIG. 17 shows a waveform indicating observed values under a peak-holdmode;

FIG. 18 shows a cross section of the double-covered optical fiber of thepresent invention;

FIG. 19 shows a cross section of the double-covered optical fiber undera measuring state;

FIG. 20 shows a cross section of the double-covered optical fiber undera measuring state.

FIG. 21 illustrates a schematic diagram of temperature measuring deviceusing the double-covered optical fiber of the present invention;

FIG. 22 is a frequency distribution showing a melt-loss in the case ofan ordinary insulation coating containing no carbon particles;

FIG. 23 is a frequency distribution showing a melt-loss in the case ofan insulation coating containing carbon particles;

FIG. 24 is a waveform diagram showing a temperature determined with theordinary insulation coating containing no carbon particles;

FIG. 25 is a waveform diagram showing a temperature determined with theinsulation coating containing carbon particles;

FIG. 26 shows a relation between the temperature of molten steel and thetemperature measured using a double-covered optical fiber with theordinary insulation coating containing no carbon particles; and

FIG. 27 shows a relation between the temperature of molten steel and thetemperature measured using a double-covered optical fiber with theinsulation coating containing carbon particles.

DESCRIPTION OF THE EMBODIMENT Embodiment-1

A temperature measuring device of the present invention for molten metalcomprises a metal-covered optical fiber which is covered with a metallictube and a radiation thermometer which is connected to the metal-coveredoptical fiber. And the metal-covered optical fiber functions as atemperature measuring element at the tip thereof.

Furthermore, a level measuring device of the present invention formolten metal comprises: a metal-covered optical fiber covered with ametallic tube, which the metal-covered optical fiber functions as atemperature measuring element at the tip thereof; a radiationthermometer connected to the metal-covered optical fiber; transfer meansto send the metal-covered optical fiber to the surface of the moltenmetal and also to retract to wind the metal-covered optical fiber fromthe surface of the molten metal; and level determination means todetermine a molten metal surface level based on a temperature changedetected by the radiation thermometer and a feed amount of themetal-covered optical fiber through means to immerse the optical fiberinto the molten steel.

According to the present invention, the tip of the metal-covered opticalfiber is immersed into a molten metal as the temperature measuringelement. An ordinary optical fiber catches fire at its coating when thetip of the optical fiber approaches the surface of the molten metal, andthe ordinary optical fiber itself easily snaps at the moment ofimmersing into the molten metal so that it can not be dipped into themolten metal. However, since the optical fiber of the present inventionis covered with a metal tube, it is immersed into the molten metalwithout damaging itself.

The metallic tube covering the optical fiber is usually made of astainless steel having a melting point ranging from 1400° to 1430° C.Consequently, when the tip of the optical fiber covered with themetallic tube is immersed into the molten metal, the metallic tube dosenot melt for several seconds and protects the optical fiber. The opticalfiber which itself forms the core has a heat durability by preparing itwith a quartz glass which has a softening point of 1600° C. or higher.

When the tip of the optical fiber covered with metallic tube is immersedinto a molten metal, the tips of the metallic tube and the optical fiberbecome the same temperature with that of the molten metal, and the tipof the optical fiber satisfies the condition of black body. Accordingly,the tip of the optical fiber is not affected by its shape, and emitsradiation light dependent only on the temperature. The radiation lightis introduced to the radiation thermometer via the optical fiber, andthe temperature of the molten metal is determined by the radiationthermometer.

In addition, when the tip of the metal-covered optical fiber coveredwith metallic tube is bent near to U-shape, a direct incident light ofradiation light of the molten metal is prevented from coming into thetip of the optical fiber before immersing the metal-covered opticalfiber into the molten metal.

Furthermore, transfer means for feeding the optical fiber sends themetal-covered optical fiber onto the surface of the molten metal toimmerse it into the molten metal, and then the means winds the releasedoptical fiber to draw up from the molten metal.

Level determination means is provided to determine the level of themolten metal based on a temperature change detected by the radiationthermometer and on an amount of feed of the optical fiber through thetransfer means to send the optical fiber.

FIG. 1 shows a schematic drawing of an embodiment of the presentinvention. According to the drawing, the temperature measuring device tomeasure the temperature of, for example, molten steel, has ametal-covered optical fiber 1 coiled on a feed drum 2, a radiationthermometer 3, and a recorder 4. The metal-covered optical fiber 1covered with a metallic tube is structured by, as shown in FIG. 2, theoptical fiber 11 made of quartz glass covered with a metallic tube 12made of stainless steel. A jelly 14 is filled between a cover layer 13on the optical fiber 11 and the metallic tube 12, at need. Themetal-covered optical fiber 1 functions as a light guide and also as atemperature measuring element. The tip of the metal-covered opticalfiber is immersed into the molten steel. A radiation thermometer 3detects and indicates the temperature of the molten metal based on theradiation light emitted from the tip of the metal-covered opticalfiber 1. The radiation thermometer 3 comprises an infrared radiationthermometer which directly determines the temperature from the luminanceoutput of radiation light, and a two-color pyrometer which determinesthe temperature by comparing the luminance at two different wavelengths.The radiation thermometer is connected to the metal-covered opticalfiber 1 via an optical fiber connector 5.

To determine the temperature of the molten steel using the temperaturemeasuring device, the tip of the metal-covered optical fiber 1 isimmersed into the molten steel. If an ordinary optical fiber having onlya coating layer is tried to immerse into the molten steel, the coatinglayer catches fire before reaching the surface of the molten steel, andthe optical fiber snaps when it touches the surface of molten steel.Since the metal-covered optical fiber 1 is protected by the metallictube 12, the optical fiber 11 and the coating layer 13 are not damagedwhile immersing them into the molten steel. The metallic tube 12covering the optical fiber 11 is made of stainless steel having amelting point ranging approximately from 1400° to 1430° C., so themetallic tube does not immediately melt even when it is immersed into ahigh temperature molten steel, and protects the optical fiber 11 forseveral seconds. Also since the optical fiber which is a core itself isprepared by quartz glass having a softening point of 1600 ° C. orhigher, it holds its original shape without melting.

Once the tip of the metal-covered optical fiber 1 is immersed into amolten steel, the tips of the metallic tube 14 and the optical fiber 11become the same temperature with that of the molten steel, and the tipof the optical fiber 11 satisfies the condition of black body. Then thetip of the optical fiber 11 emits the radiation light which depends onlyon the temperature. The radiation light is sent to the radiationthermometer 3 via the metal-covered optical fiber 1. The radiationthermometer 3 calculates the temperature based on the wavelength ofreceived radiation light and determines the temperature of the moltensteel to indicate it, and sends the temperature output signal to therecorder 4 to store the value. The immersion of the tip of themetal-covered optical fiber 1, which configuration gives a small heatcapacity, allows to instantaneous follow up of the tip to thetemperature of the molten steel and allows to give a quick and accuratemeasurement of the temperature of the molten steel. A direct currentstyle radiation light sent to the radiation thermometer 3 can beprocessed by a photo-chopper to convert into an intermittent alternatecurrent signal and to amplify it, then the amplified alternate currentsignal can be detected by synchronizing with the photo-chopper toachieve a stable amplification.

When the tip of the metal-covered optical fiber 1 is kept immersed intothe molten steel for a long time after detecting the temperature of themolten steel, the coating layer 13 of the optical fiber 11 gasifies inthe high temperature environment. Then, the generated gas ejects fromthe tip of the optical fiber, and ignites if oxygen exists. To preventsuch an accident, the tip of the metal-covered optical fiber 1 is drawnup from the molten steel immediately after measuring the temperature ofthe molten steel. Then, the tip once used as the temperature measuringelement is cut off before next temperature measurement cycle, and thefresh tip is immersed into the molten steel at the next measuring cycle.In this way, a coil of the metal covered optical fiber 1 is successivelyused to measure the molten steel temperature.

Now, some examples of temperature measurement for molten steel using atemperature measuring device structured as described above will begiven.

FIG. 3 illustrates a schematic diagram of temperature measurement for amolten steel in a mold of a continuous casting machine using thetemperature measuring device described above. As seen in the figure, inthe continuous casting machine, the molten steel 33 is poured into amold 34 via the tundish 31 and the immersion nozzle 32. Normally, apowder 35 is spread on the molten steel 33 in the mold 34. To measurethe temperature of the molten steel in the mold 34, the metal-coveredoptical fiber 1 covered with metallic tube coiled around the feed drum 2is sent out at a certain speed using the fiber transfer means 36 ineither a continuous manner or an intermittent manner, and the tip of themetal-covered optical fiber 1 is immersed into the molten steel 33through the powder 35. In this manner, the temperature is determined bythe radiation thermometer 3 while feeding the tip of the metal-coveredoptical fiber 1, and the determined temperature is recorded in therecorder 4. Then, as seen in FIG. 4, the characteristic curve of a fiberfeeding time and a temperature change, the temperature shows a suddenrise to t₁ at the time when the tip of the metal-covered optical fiber 1is immersed into the molten steel 33, and the temperature saturates at Tof the molten steel 33. The detection of the saturated temperatureallows the measurement of the temperature T of the molten steel 33.

When the feed speed of the fiber transfer means 36 is controlled duringfeeding of the tip of metal-covered optical fiber 1, the temperature T₁of the molten powder 35 is determined clearly separately from thetemperature T of the molten steel 33, as shown in FIG. 5 of temperaturechange characteristic graph. And it is also possible to determine atemperature distribution of the molten steel 33 in its depth directionas well as the temperature T₁ of the molten powder 35. Nevertheless, aprolonged immersion of the metal-covered optical fiber results in themelting of the metallic tube 12 to melt the tip of the metal-coveredoptical fiber 1. Therefore, after completing the measurement, the fibertransfer means 36 starts its operation in reverse direction to draw upthe metal-covered optical fiber 1 at a high speed to wind around thefeed drum 2 to prepare for the next measurement. With the repetition ofthe procedure described above, the temperature of the molten steel 33 inthe mold 34 can be accurately determined.

During the measurement of the molten steel 33, a shortened dip depth ofthe metal-covered optical fiber I into the molten steel 33 and ashortened immersion time reduce the consumption of the metal-coveredoptical fiber 1. The immersion time depends on the time for thetemperature measurement. The time necessary for measuring temperature isdetermined by a response time of the radiation thermometer 3, and isindependent of the transmission of radiation light. When a semiconductordevice is used as a light receptor of the radiation thermometer 3, theresponse time of the thermometer is in the order of msec., which meansthat the consumption of the metal-covered optical fiber 1 is as small asapproximately 1 cm in every measurement. As a result, the repeatedmeasurement of the temperature of the molten steel 33 is performed at alow cost.

If the temperature determined by the repeated measurements becomes tobelow a specified level, then the process can be stabilized by enhancingthe heating with a plasma torch 37 to raise the temperature of thetundish 31.

The temperature distribution within the mold 34 is also determined bymeasuring the temperature of the molten steel 33 while changing themeasuring points in the mold 34 and, as seen in FIG. 6, by using amulti-points thermometer combined with the radiation thermometer 3 andconnected to more than one metal-covered optical fiber 1 via an opticalswitch 38. The obtained data of temperature distribution within the mold34 allows to predict a flow rate distribution of molten steel comingfrom the nozzle 32, which information contributes to the stableoperation of the continuous casting machine.

The above-described example deals with the measurement of temperature ofthe molten steel 33 in a mold 34. The level of the molten steel 33 canbe measured also with the temperature of the molten steel 33 within themold 34.

FIG. 7 illustrates a scheme for the measurement of temperature and alevel of molten steel 33 in the mold 34 of a continuous casing machine.As seen in the figure, the level of the molten steel 33 in the mold 34at a steady state is determined by a level detector 41 which comprisesan eddy current distance detector. The tip of the metal-covered opticalfiber 1 covered with metallic tube is placed at an initial leveldetection point L₁ in the mold 34. The temperature output terminal ofthe radiation thermometer 3 is connected to the level detection means42. The level detection means 42 identifies the time when the moltensteel 33 in the mold 34 reaches the initial level detecting point L1. Athreshold value TH corresponding to the initial level detection point L₁is entered to the level detection means 42 in advance.

When a nozzle stopper 43 is turned to open, the molten steel 33 in thetundish 31 is poured onto a dummy bar 44 through a nozzle 32, and alevel of the molten steel 33 in the mold 34 rises. When the molten steel33 approaches to the tip of the metal-covered optical fiber 1 at theinitial detection point L₁, the radiation light emitted from the moltensteel 33 and entered the metal-covered optical fiber 1 increases, andthe temperature output of the radiation thermometer 3 graduallyincreases as shown in FIG. 8. The temperature output is sent to thelevel detection means 42. The level detection means 42 compares thetemperature of the molten steel 33 with the threshold value TH. When themolten steel 33 touches the tip of the metal-covered optical fiber 1,the temperature output of the radiation thermometer 3 shows a rapid riseindicating the temperature of the molten steel 33. When the temperatureinformation sent from the radiation thermometer 3 exceeds the thresholdvalue TH, the level detection means 42 judges that the level of moltensteel 33 reaches the initial level detection point L₁ , and sends theinitial level detection signal to nozzle stopper point control means 45.On receiving the initial level detection signal, the nozzle stopperpoint control means 45 adjusts the position of the nozzle stopper 43 tocontrol the flow rate of the molten steel to that of the second stage.In addition, the initial level detection signal actuates the draw-out ofthe dummy bar 44 and the take-up of the metal-covered optical fiber 1automatically. When the level of molten steel 33 in the mold 34 entersin a measuring range of an eddy current level detector 41, the levelcontrol of the molten steel is conducted using the output of the leveldetector 41. The repeated feeding and drawing up of the metal-coveredoptical fiber 1 allows to determine the temperature of the molten steel33 in the mold 34.

Since the setting position of the tip of the metal-covered optical fiber1 is arbitrarily selected at the detection of the initial leveldetection point L₁ in the molten steel 33 in the mold 34, the initiallevel detection point L₁ can be set in a wide range.

Accordingly, an automatic start which was difficult in prior art can berealized.

Also since the diameter of the metal-covered optical fiber 1 is as thinas in a range of from 1 to 2 mm, it is applicable for a continuouscasting of small cross sectional work such as billet, and the industrialeffect is significant.

The description given above uses an example of a single metal-coveredoptical fiber 1. However, as shown in FIG. 6, when the tips of more thanone metal-covered optical fiber 1 are positioned at different levels,the level change of the molten steel 33 during the initial stage can bedetected.

The description given above deals with the case for measuring atemperature and level of the molten steel 33 in the mold 34 of acontinuous casting machine. The temperature and level of the moltensteel 33 in a tundish 31 can also be measured.

FIG. 9 illustrates a scheme for measuring the temperature and level ofthe molten steel 33 in a tundish 31. According to the figure, themetal-covered optical fiber 1 is inserted through a measurement hole 51opened on a lid of the tundish 31 using fiber transfer means 36 at aconstant speed, continuously or intermittently. When the tip of themetal-covered optical fiber 1 reaches the surface of the molten steel33, the temperature output of the radiation thermometer 3 shows a suddenchange, as seen in FIG. 4, and the temperature saturates at thetemperature of the molten steel 33. The sudden temperature change isdetected by the level detection means 42. Then the level detection means42 calculates to determine the level of the molten steel 33 in thetundish 31 based on the information of feed length of the metal-coveredoptical fiber 1, which information is transmitted from the feed controlmeans 52. The level detection means 42 sends the computed leveldetection signal to level control means 53, and also sends it to thefeed control means 52 to stop the feeding action of the fiber transfermeans 36 to stop the immersing action of the metal-covered opticalfiber 1. Once the temperature and level of the molten steel 33 aredetected, the metal-covered optical fiber 1 is wound back by the fibertransfer means 36 to prepare for the next measurement cycle.

Following the procedure described above, the temperature and the levelof the molten steel 33 in the tundish 31 are detected at the same time.Accordingly, the detection function significantly contributes to theautomatization and stabilization of the tundish operation.

The above described examples deal with the tip of the metal-coveredoptical fiber 1 as the temperature measuring element in a straightshape. Nevertheless, as shown in FIG. 10, the tip of the metal-coveredoptical fiber 1 may be bent in near U-shape or can be bent to 90 degreesor more to form a temperature measuring element 1a. Such a bent tipprevents direct incident radiation light emitted from the molten steel33 into the tip during feeding stage of the metal-covered optical fiber1 approaching toward the surface of the molten steel 33. That type ofbent tip allows a more rapid temperature output rise in the radiationthermometer when the tip is immersed into the molten steel than the caseof straight tip, which results in a high accuracy detection of thetemperature and the level of the molten steel 33.

The above described examples deal with the case for measuring thetemperature and level of the molten steel 33 in a continuous castingprocess. The preferred mode of the present invention also measures thetemperature and the level of the molten metal in other apparatus, andthe invention is applicable for a wide range.

As described in detail above, the present invention employs the tip ofthe metal-covered optical fiber to immerse into a molten metal.Accordingly, the optical fiber itself as the core is immersed into themolten metal without suffering damage.

In addition, the present invention secures the heat resistance ofoptical fiber by protecting the optical fiber with a metallic tubecovering thereof and by forming the optical fiber with a quartz glass.Consequently, the present invention allows a stable measurement oftemperature of the molten metal.

When the tip of the metal-covered optical fiber is immersed into amolten metal, the cover metal tube and the tip of the optical fiberbecome the same temperature with that of the molten metal.

As a result, the emitted radiation light depends only on thetemperature, so the temperature of the molten metal can be determinedrapidly and precisely.

By adding level detection means, the level of the molten metal is alsodetermined based on the data of the feed length of the metal-coveredoptical fiber and the temperature change measured by a radiationthermometer. The level information largely contributes to theautomatization and stabilization of process operation.

Furthermore, by bending the tip of the metal-covered optical fiber tonear U-shape, the radiation light of the molten metal can be preventedfrom directly entering to the tip of the optical fiber at the immersingstage of the metal-covered optical fiber, which assures an accuratedetection of temperature and level of the molten metal.

Since the radiation light is transmitted through the metal-coveredoptical fiber, a stable measurement is carried out without sufferingnoise effect.

Embodiment-2

A consumable optical fiber temperature measuring device of the presentinvention comprises an optical fiber covered with a protective tube andan insulation coating on the outer surface of the protective tube toform a double-covered optical fiber. A tip of the double-covered opticalfiber is used as a temperature measuring element, the optical fiber hasa temperature at which the optical fiber is consumed point higher thanthe temperature of a molten metal being measured, and the protectivetube and the insulation coating have a heat-resistant temperature lowerthan the temperature of the molten metal to be measured.

The protective tube is preferably a metallic tube having a higher heatresistant temperature than that of the insulation coating.

The consumable optical fiber temperature measuring device of the presentinvention further has optical fiber transfer means which sends thedouble-covered optical fiber intermittently to a surface of the moltenmetal, and has correction means which corrects a light energy obtainedat an end of the optical fiber using an initial length of the opticalfiber specified in advance, a loss per unit length of the optical fiber,and a change of feed length of the optical fiber detected by the opticalfiber transfer means in every measuring cycle.

According to the present invention, the tip of the double-coveredoptical fiber, covered with a protective tube and an insulation coating,is immersed into the molten metal as the temperature measuring element.When the tip of the double-covered optical fiber is immersed into themolten metal, the optical fiber is not damaged because it is coveredwith a protective tube.

In addition, the optical fiber has a temperature at which the opticalfiber is consumed point higher than the temperature of molten metalbeing measured, and both the protective tube and the insulation coatinghave a heat resistant temperature lower than the temperature of themolten metal. Accordingly, when a double-covered optical fiber isimmersed into the molten metal, the tip of the double-covered opticalfiber firstly burns out the outside insulation coating, then theprotective tube burns out to expose the optical fiber. The exposedoptical fiber receives the energy of incident light and detects thetemperature of the molten metal.

In this manner, the protective tube is formed by a metallic tube havinga higher heat resistant temperature than that of the insulation coatingto give a delay time between the burning of the insulation coating atthe tip and the melting of the protective tube. The successive burningand melting protects a tip of the optical fiber for a certain period toensure the uniform temperature on the whole region of the tip of theoptical fiber.

Accordingly, when the double-covered optical fiber is immersed into themolten metal, the tip of the optical fiber is melted to degrade andconsumes. As a result, if the double-covered optical fiber iscontinuously immersed into the molten metal, then the consumption of theoptical fiber increases. Therefore, the double-covered optical fiber isintermittently immersed into the molten metal while sending andretracting to wind the optical fiber using optical fiber transfer means.

When the tip of the double-covered optical fiber is successivelyconsumed, the total length of the optical fiber become short, and thetransmission loss of the optical fiber decreases, which results in anincrease of the energy of light taken out at the end of the opticalfiber to yield an error. To compensate the bad effect, correction meansis provided to correct a light energy obtained at an end of the opticalfiber using an initial length of the optical fiber specified in advance,a loss per unit length of the optical fiber, and a change of feed lengthof the optical fiber detected by the optical fiber transfer means inevery measuring cycle.

FIG. 11 shows a schematic drawing of an example of the presentinvention. According to the drawing, the temperature measuring device todetermine, for example, the temperature of molten steel comprises adouble-covered optical fiber 101 coiled around a feed drum 102, opticalfiber transfer means 103, feed control means 104, and a signal processor105.

The double-covered optical fiber 101 functions as a light wave guide andthe temperature measuring element. The optical fiber 111 which itselfforms a core is made of GI fiber, a quartz glass. As seen in FIG. 12,the outside surface of the optical fiber 111 is coated by a UVcross-linked plastic material, which is then covered with a protectivetube 112 made of stainless steel. The protective tube 112 is furthercovered with an insulation coating 113 of synthetic resin such aspolyethylene or of glass fiber.

The optical fiber transfer means 103 sends the double-covered opticalfiber 101 coiled around the feed drum 102 continuously or intermittentlyand immerses the tip of optical fiber 101 into the molten steel 107 fromthe top of the layer of powder 106. After a predetermined time haspassed, the optical fiber transfer means 103 retracts the double-coveredoptical fiber 101 to take up from the molten steel 107 by winding it.The feed control means 104 actuates intermittently while controlling asending amount of optical fiber through the optical fiber transfer means103.

A signal processor 105 comprises a light detector 151, correction means152, memory means 153, a peak-hold circuit 154, recording means 155, andindication means 156. The light detector 151 generates electric signalsproportional to the incident light power. The input section of the lightdetector 151 is connected to the end of the double-covered optical fiber101. The memory means 153 stores the initial length Lo of thedouble-covered optical fiber 101 and the loss per unit length. Thecorrection means 152 corrects the effect of change of transmission lossgenerated in the optical fiber 111 on the electric signals sent from thelight detector 151 in every measurement cycle using the initial lengthL₀ of the double-covered optical fiber and the loss per unit length,both of which are stored in the memory means 153, and using the changeΔLn of the feed length of the double-covered optical fiber 101, whichchange is sent from the control means 104 at every measurement cycle, orthe consumption of the tip of the double-covered optical fiber 101. Thepeak-hold circuit 154 detects the peak value of signals sent from thecorrection means 152 and holds the peak value for a specified time.

To conduct a temperature measurement of the molten steel 107 using atemperature measuring device having the structure described above, theinitial length L₀ of the double-covered optical fiber 101 coiled aroundthe feed drum 102 and the loss per unit length are measured in advanceand stored in the memory means 153. Then, the double-covered opticalfiber 101 is fed by the optical fiber transfer means 103 to immerse thetip of the double-covered optical fiber 101 into the molten steel 107through the powder 106. If an ordinary optical fiber is tried to immerseinto the molten steel 107, when the tip of the ordinary optical fiberapproaches the surface of the molten steel 107, the coating layer of theordinary optical fiber catches fire, and the ordinary optical fiberitself snaps while passing through the powder 106. However, since thedouble-covered optical fiber 101 is covered with a protective tube 112made of stainless steel and with an insulation coating 113, theinsulation coating 113 requires a heat of vaporization when the tip ofthe double-covered optical fiber 101 passes through the powder 106, andit takes a time until the insulation coating 113 is completelyvaporized. As a result, the protective tube 112 is not damaged while theoptical fiber is stably immersed into the molten steel 107.

When the tip of the double-covered optical fiber 101 is immersed intothe molten steel 107 having a temperature of, for example, 1500° C. ormore, the temperature at the tip of the double-covered optical fiber 101shows a sudden increase, and the strength of the insulation coating 113at the tip suddenly decreases to begin the firing to degrade, and theprotective tube 112 gradually exposes from the tip thereof. Since theprotective tube 112 is formed by a stainless steel tube having a meltingpoint of approximately 1400° to 1430° C., if the protective tube 112 isimmersed into the molten steel 107 of 1500° C. or more, it begins tomelt and the optical fiber 111 gradually exposes as seen in the crosssection of FIG. 13.

A light depending on the temperature of the molten steel 107 immediatelyenters the tip of the exposed optical fiber 101. The light is sent tothe signal processor 105 through the double-covered optical fiber 101,where the light is converted to electrical signals, which are thenconverted to temperature.

Since the optical fiber 111 exposes from its tip when the tip of thedouble-covered optical fiber 101 is immersed into the molten steel 107while waiting for a time lag to melt the insulation coating 113 and theprotective tube 112, the tip of the optical fiber 111 can be maintainedat a specified depth in the molten steel 107. Also since the opticalfiber 111 is prepared by a quartz glass having a softening point ofapproximately 1600° C. which is higher than the temperature of themolten steel 107, the optical fiber 111 keeps its original shape for acertain time after the exposure. Consequently, the internal temperatureof the molten steel 107 is determined promptly and accurately.

If the tip of the optical fiber 111 which has been exposed is keptimmersed in the molten steel 107, then the coating layer of the opticalfiber 111 is gasified in the ambient high temperature to gradually meltthe optical fiber 111 from the tip to consume. Therefore, immediatelyafter measuring the temperature of the molten steel 107, the tip of thedouble-covered optical fiber 101 is drawn out from the molten steel 107using the optical fiber transfer means 103 to reduce the consumption ofthe optical fiber 111. The tip of the drawn-out double-covered opticalfiber 101 increases its outside diameter nearly uniformly along thelength of the optical fiber by burning out as shown in FIG. 14, and thetip of the protective tube 112 melts to cover the tip of the opticalfiber 111.

In this manner, the protective tube 112 and the insulation coating 113remain at the tip of the double-covered optical fiber 101, and theprotective tube 112 covers the tip of the optical fiber 111, so theoptical fiber 101 easily penetrates the powder 106 at the next measuringcycle and immerses stably into the molten steel 107. Furthermore, theportion of the protective tube 112 which covers the tip of the opticalfiber 111 immediately melts after immersing into the molten steel 107 toexpose the double-covered optical fiber 111 and to prepare the measuringmode. As a result, the tip of the double-covered optical fiber 101 isnot needed to machine in every measuring cycle and is applicable fordetermining the molten steel temperature repeatedly.

The following is an example of the state of tip of the double-coveredoptical fiber 101 under the measurement of the temperature of the moltensteel 107.

The applied double-covered optical fiber 101 consisted of GI fiber, aquartz glass. The 50/125/250 optical fiber 111 was coated with a UVcross-linked plastic material and further was covered with a protectivetube made of stainless steel to form a metallic tube covered opticalfiber having an outside diameter of 1.2 mm. The outside surface of theprotective tube was coated with an insulation 113 made of polyethyleneresin or made of a glass fiber to prepare a double-covered optical fiberhaving an outside diameter of 4 mm.

The tip of the prepared optical fiber was immersed into the molten steel107 to a depth of approximately 200 mm, and held it immersing for 2 sec.The intermittent repetition of the immersion was carried out todetermine the temperature of the molten steel 107. After everymeasurement cycle, the tip of the immersed double-covered optical fiberwas checked. The tip immersed into approximately 200 mm depth of themolten steel 107 remained approximately 100 mm of the tip 101a in aconical shape after taken out from the molten steel 107, which is shownin FIG. 14. The state indicated that, during the measurement for 2 sec,the tip of double-covered optical fiber 101 lost approximately 100 mm inits length by melting, but the tip was held in the molten steel 107 tomeasure the temperature of the inside of the molten steel 107, not ofthe temperature of the powder 106 on the molten steel 107.

As shown in FIG. 15, the difference of measured peak temperature in ntimes gave approximately 1° C., and the observed values in more than onemeasurement using a continuous thermometer followed well the actualtemperature of molten steel 107 within about 1° C. FIG. 15 indicates asudden fall of the temperature after reached to a peak value. Thephenomenon shows that the tip of the optical fiber 101 is exposed tomelt out to expose a fresh tip portion.

FIG. 16 shows an observed result when the tip of the double-coveredoptical fiber 101 was immersed into the molten steel 107 to a depth ofapproximately 200 mm to hold for 1 sec. and when the immersion wasrepeated intermittently. The repeated immersion of the tip of thedouble-covered optical fiber 101 into the molten steel 107 for 1 sec.accurately followed the actual temperature change of the molten steel107 determined by a continuous thermometer, and the tip length of thedouble-covered optical fiber 101 consumed in every measurement was in arange of from 10 to 20 mm. Consequently, an accurate temperaturemeasurement within a short time and a minimized melt consumption lengthof the double-covered optical fiber 101 were achieved.

Repeated immersion of the double-covered optical fiber 101 into themolten steel 107 to determine the temperature of the molten steel 107 asdescribed above shortens the total length of the double-covered opticalfiber 101 because of the repeated melting out at the tip ofdouble-covered optical fiber 101. In this way, when the total length ofthe double-covered optical fiber 101 is shortened, the transmission lossof the optical fiber 111 becomes small, and the light energy generatedfrom the end of the optical fiber 111 increases, which yields an errorof observed value. To cope with the phenomenon, the light energygenerated from the end of the optical fiber 111 is sent to the lightdetector 151 of the signal processor 105, and the light energy enteredthe light detector 151 is converted into an electric current signal, In,proportional to the power of the entered energy and is sent to thecorrection means 152. The correction means 152 corrects the receivedelectric current signal, In, using the initial length, Lo, of theoptical fiber 111 and the loss, α, per unit length, both of which arestored in the memory means 153, and using the change of feed length, Ln,of optical fiber in every measurement, which is sent from the feedcontrol means 104. The corrected value, Inc, which removed the change oftransmission loss, is sent to the peak-hold circuit 154. The peak-holdcircuit 154 maintains the received peak value of the correction value,Inc, for a predetermined period and sends the peak value to theindication means 156 and also records the peak value in the recordingmeans 155.

In such a manner as described above, the change of transmission loss inthe optical fiber 111 is corrected by correction means 152. Accordingly,even when the tip of the double-covered optical fiber 101 is consumed,the generation of error of measured temperature is prevented, and thestable temperature measurement of molten steel 107 for a long period isensured. Also when the peak value of the corrected observed value, Inc,is held for a specified period, 1 sec. for example, the observedtemperature values can be indicated surely on the indication means 156and the recording means 155, which is illustrated in FIG. 17.Furthermore, by detecting and holding the peak value of the observedvalue Inc, the time for immersing the tip of the double-covered opticalfiber 101 into the molten steel 107 is reduced, which in turn enables along period of measurement with a single double-covered optical fiber101.

The above-described example deals with a case that uses the opticalfiber 111 made of quartz glass and the protective tube 112 made ofstainless steel to measure the temperature of the molten steel as highas 1500° C. or more. Nevertheless, a temperature at around 1000° C. maybe measured using the optical fiber 111 made from a multi-componentglass having a softening temperature of approximately 1000° C. andcovering the optical fiber 111 with a synthetic resin insulation 113.

As detailed above, the present invention assures a stable immersion ofthe optical fiber into the molten metal without damaging the opticalfiber while functioning the tip of the optical fiber as the temperaturemeasuring element because the optical fiber is covered with theprotective tube.

In addition, since the employed optical fiber has a temperature at whichthe optical fiber is consumed higher than the temperature of the moltenmetal being measured and since the heat-resistant temperature of boththe protective tube and the insulation coating is lower than thetemperature of the molten metal, there is a time lag between the time ofimmersing the double-covered optical fiber into the molten metal and thetime of exposing the tip of the optical fiber, which ensures the holdingof the tip of optical fiber in the molten metal for a prompt andaccurate measurement of the inside temperature of the molten metal.

Furthermore, since the protective tube is prepared with a metallic tubehaving higher heat resistant temperature than that or the insulationcoating, a delay time is secured after the burn-out of the insulationcoating at the tip until the burnt-out of the protective tube, whichprotects the tip of the optical fiber and which makes the whole area ofthe tip of the optical fiber uniform in temperature.

Also since the optical fiber is protected by the protective tube and theinsulation coating, the amount of melt-out at the tip is minimized whenthe double-covered optical fiber is immersed into the molten metal.Therefore, even when the double-covered optical fiber is immersedintermittently into the molten metal, the consumption of the tip ofdouble-covered optical fiber is reduced. As a result, a singledouble-covered optical fiber performs a long period of measurement.

With a correction of the change of transmission loss caused from theconsumption of the tip of double-covered optical fiber, the generationof error in the observed temperature can be prevented even when the tipof double-covered optical fiber is consumed. The correction also assuresa stable and long period of temperature measurement of molten metal.

Embodiment-3

The double-covered optical fiber of the present invention has aprotective tube on the optical fiber, and further has an insulationcoating covering the protective tube, which insulation coating containsparticles having a melting point higher than the temperature of a targetmaterial.

The particles of high melting point use carbon particles. Furthermore,the double-covered optical fiber of the present invention has a metallicprotective tube thereon, and further has an insulation coatingcomprising cellulose covering the protective tube.

The insulation coating is made of paper.

For example, carbon (graphite) is an element of the highest meltingpoint group as high as above 3,500° C. When an insulation materialcontaining the carbon particles as the additive is immersed into amolten metal, the carbon particles do not burn because they can notreceive oxygen and leave a charcoal layer on the protective tube afterthe insulation material melted out in the molten metal. The remainedcharcoal layer gives an insulation effect to suppress the melt-loss ofthe protective tube. In addition, a metal having a high temperaturedurability supports to maintain the flexural strength of the opticalfiber.

In this way, an insulation coating containing particles as the additivehaving a temperature of melting point higher than the temperature of thetarget material reduces the consumption of the optical fiber andmaintains the strength of the protective tube, which enables todetermine the temperature inside of (deep in) the molten metal.

Also when an insulation material comprising cellulose is immersed into amolten metal, the heat-receiving condition without accompanyingoxidation is established. As a result, a carbon structure is left afterthe melting-out of the insulation material to remain on the surface ofthe protective tube as charcoal. The charcoal gives an effect of heatinsulation to suppress the damage of the protective tube. In addition, ametal having a high temperature durability supports to maintain theflexural strength of the optical fiber.

Following the procedure, the insulation coating comprising cellulosecomponent reduces the consumption of the optical fiber and maintains thestrength of the protective tube to enable the measurement of inside of(deep into) the molten metal.

Example 1

FIG. 18 shows a cross section of a fiber of Example 1 of the presentinvention. The reference number 201 designates the double-coveredoptical fiber. The double-covered optical fiber 201 comprises a 50/125optical fiber 211 made of GI fiber, a quartz glass, coated withpolyimide, a protective tube 212 made of stainless steel having 1.4 mmof outer diameter and 1 mm of inner diameter covering the optical fiber211, and an insulation coating 213 made from polyethylene resincontaining carbon particles as the additive at a rate of approximately3% covering the surface of the protective tube 212.

For example, as illustrated in FIG. 21, a temperature measuring deviceto determine the temperature of the molten steel is structured by thedouble-covered optical fiber 201 which is coiled around a feed drum 202and which is used as both the light guide and the temperature measuringelement, optical fiber transfer means 203, and a signal processor 204.Purging with an inert gas 207 is applied surrounding the double-coveredoptical fiber near to the molten steel surface to perform a maximumeffect of the heat-resistant covering of the double-covered opticalfiber 201 and to prevent burning the cover material.

With the temperature measuring device having the above-describedstructure, the temperature of the molten steel is determined. When thetip of the double-covered optical fiber 201 is immersed into the moltensteel 205 through the powder 206, the insulation coating 213 meltsduring the tip of the double-covered optical fiber 201 passes throughthe powder 206 while leaving a charcoal layer on the surface of theprotective tube 212 because the optical fiber 211 is covered with theprotective tube 212 made of stainless steel and with the insulationcoating 213 comprising polyethylene containing carbon particles as theadditive at a rate of approximately 3%. The remained charcoal layergives an effect of heat insulation to protect the protective tube 212from heat, so the optical fiber 211 is possible to immerse into themolten steel 205 stably.

On the contrary, in the case of an insulation coating made from ordinarypolyethylene which contains no carbon particle, the insulation coatingmelts out on passing through the powder to expose the protective tube212. As a result, compared with the case of the present example whichuses the insulation coating 213 made from polyethylene containing carbonparticles as the additive, the speed of melting the protective tube ishigh, and that type of the optical fiber 211 can not immerse stably intoa sufficient depth. This is because the protective tube degraded itsflexural strength can not endure the pressure of the molten steel 205and can not immerse into the molten steel 205, and because the amount ofmelt-loss is excessive even when the optical fiber 211 is immersed intothe molten steel 205.

When the double-covered optical fiber 201 using the insulation coating213 containing carbon particles as the additive is immersed into themolten steel at 1500° C. or more, the temperature of the tip of thedouble-covered optical fiber 201 shows a sudden rise, and the insulationcoating 213 at the tip is melted out to leave a thin charcoal layer onthe surface of the protective tube 212. Since the protective tube 212 isformed by a stainless steel having a melting point in an approximaterange from 1400° to 1430° C., the area where is not insulated by thecharcoal insulation layer receives heat to gradually melt from the tip,and the optical fiber 211 gradually exposes from the tip as shown in thecross section in FIG. 19.

The tip of the optical fiber 211 which has been exposed immediatelyreceives the light depending on the temperature of the molten steel 205The incident light is sent to the signal processor 204 via thedouble-covered optical fiber 211 and is converted to temperature.

When the insulation coating 213 containing carbon particles as theadditive is used, there appears a time lag between the time of immersingthe tip of the double-covered optical fiber 201 into the molten steel205 and the time of melting the protective tube 212 until the opticalfiber 211 exposes. So the tip of the optical fiber 211 is secured at aspecified depth in the molten steel 205. Since the optical fiber 211 ismade from a quartz glass having a softening point at approximately 1600°C. which is higher than the temperature of the molten steel 205, theoptical fiber 211 maintains the original shape for a certain period evenafter it is exposed. Therefore, the internal temperature of the moltensteel 205 can be determined promptly and accurately.

After the measurement, the double-covered optical fiber 201 is drawn outfrom the molten steel 205. The tip of the protective tube 212 is meltedto cover the tip of optical fiber 211, which is shown in FIG. 20. Insuch a manner, the protective tube 212 and the insulation coating 213are remained at the tip of the double-covered optical fiber 201, and theprotective tube 212 covers and protects the tip of optical fiber 211.Accordingly, the optical fiber 211 can be immersed deep into the moltensteel 205 at next measuring cycle.

An example is referred to observe the state of the tip of thedouble-covered optical fiber 201 on measuring the temperature of themolten steel 205. The double-covered optical fiber 201 comprises a50/125 optical fiber 211 made of GI fiber of a quartz glass coated withpolyimide, a protective tube 212 made of stainless steel having 1.4 mmof outer diameter and 1 mm of inner diameter covering the optical fiber211, and an insulation coating 213 made from polyethylene resincontaining carbon particles as the additive at a rate of approximately3% covering the surface of the protective tube 212.

The tip of double-covered optical fiber 201 was immersed into the moltensteel 205 at 1550° C. to a depth of approximately 200 mm to hold itthere for 1 sec. The measurement was repeated intermittently. In everymeasurement, the shape of the tip of the double-covered optical fiber201 was observed to find that the tip immersed into the molten steel 205to a depth of approximately 200 mm became a shape as shown in FIG. 20,while leaving approximately 160 mm long tip. During 1 sec. ofmeasurement, it was found that the tip of the double-covered opticalfiber 201 lost about 40 mm long, but the tip was held in the moltensteel 205, not in the powder 206 above the molten steel, and that theinside temperature of the molten steel 205 was measured.

Next, to clarify an effect of the present example, the double-coveredoptical fiber 201 of the example using the insulation coating 213composing polyethylene containing about 3% of carbon particles as theadditive was compared with the double-covered optical fiber using aninsulation coating composing ordinary polyethylene containing no carbonparticle.

FIG. 22 and FIG. 23 show the melt-loss of the optical fiber when the tipof the double-covered optical fiber was repeatedly immersed into themolten steel 205 at 1550° C. to a depth of 200 mm to hold it there for 1sec. FIG. 22 indicates the melt-loss in the case of an insulationcoating of ordinary polyethylene without containing carbon particleadditive. FIG. 23 shows the melt-loss in the case of an insulationcoating polyethylene containing approximately 3% of carbon particles asthe additive.

The melt-loss of the case using an insulation coating of ordinarypolyethylene containing no carbon particle was approximately 120 mm. Themelt-loss of the case using the insulation coating of polyethylenecontaining carbon particles at approximately 3% as the additive wasapproximately 40 mm, which showed a significant difference in each ofthe cases. The comparison confirmed the superiority of the polyethylenecontaining carbon particles as the additive.

FIG. 24 and FIG. 25 show an observed waveform when the tip ofdouble-covered optical fiber was repeatedly immersed into the moltensteel 205 to a depth of approximately 200 mm and held it at the placefor 1 sec. FIG. 24 is drawn in the case of the insulation coating ofordinary polyethylene without containing carbon particle additive. FIG.25 shows the melt-loss in the case of the insulation coating ofpolyethylene containing approximately 3% of carbon particles as theadditive. In FIG. 24, there observed several times of sudden decrease ofmeasured value. The phenomenon indicates the temperature change occurredat the time of melting out of the tip of the exposed optical fiber toexpose a new tip portion. On the other hand, FIG. 25 shows a stableplateau, which indicates that the optical fiber 211 was well protectedand that the measurement was conducted at a stable position.

Then, the tip of the double-covered optical fiber was immersed into themolten steel 205 which was varied its temperature in a range from 1500°to 1600° C. to a depth of approximately 200 mm to hold there for 1 sec.The intermittently repeated measurement was carried out, and the resultsare given in FIG. 26 and FIG. 27. FIG. 26 is drawn in the case of theinsulation coating of ordinary polyethylene without containing carbonparticle additive. FIG. 27 shows the melt-loss in the case of theinsulation coating polyethylene containing approximately 3% of carbonparticles as the additive.

Referring to FIG. 26, there often appeared a temperature indicationlower than the actual molten steel by nearly 20° C. The presumed causeof the phenomenon is that the tip of the double-covered optical fiber201 lost its insulation coating before reaching to a sufficient depthduring the process of immersing the double-covered optical fiber 201into the molten steel 205 and that the protective tube 212 was directlyexposed to the molten steel 205 to lose its strength and that the tipcould not reach a sufficient depth and the correct temperature of moltensteel 205 was not able to be measured.

To the contrary, in the case of using the insulation coating 213comprising polyethylene containing carbon particles as the additive,which is shown in FIG. 27, there attained nearly completely stablemeasurement of correct temperature.

With the above temperature measurement results, the superiority of theinsulation containing carbon particles as the additive was confirmed.

The above example uses the insulation coating comprising polyethylenecontaining carbon particles at about 3% as the additive. Nevertheless,an insulation coating of polyethylene containing carbon particles atabout 5% gives similar effect with the 3% case. If the bendingcharacteristics of the optical fiber on coiling around the drum is notrequired to consider, the content of the carbon particles may beincreased further.

Example 2

Example 2 used a heat-shrink tube made of polyethylene containing carbonparticles as the additive as an insulation coating of the double-coveredoptical fiber. The following is the description of the structure of thedouble-covered optical fiber of Example 2.

The double-coated optical fiber comprised a 50/125 optical fiber 211made of GI fiber of a quartz glass coated with polyimide, a protectivetube 212 made of stainless steel having 1.4 mm of outer diameter and 1mm of inner diameter covering the optical fiber 211, and a heat-shrinktube made from polyethylene resin containing carbon particles as theadditive covering the surface of the protective tube 212 to form thedouble-covered optical fiber with outside diameter of approximately 4mm.

The tip of the double-covered optical fiber was immersed into a moltensteel at a temperature ranging from 1400° to 1600° C. to a depth ofapproximately 200 mm to hold it there for 1 sec. The measurement wasrepeated intermittently. In every measurement, the shape of the tip ofdouble-covered optical fiber was observed to find that a tip having ashape similar to FIG. 20 remained and that the melt-loss at the tip ofthe double-covered optical fiber was approximately 30 mm, which wassimilar effect with the case using an insulation coating of polyethylenecontaining approximately 3% of carbon particles and that the tip wasimmersed to a sufficient depth to determine correct temperature of themolten steel using a temperature measuring device employing thedouble-covered optical fiber.

From the above results, also with a double-covered optical fiber usingan insulation coating of polyethylene heat shrink tube, it was confirmedthat the temperature of the molten steel at a sufficient depth can bemeasured with a reduced consumption of the optical fiber in everymeasurement.

Example 3

Examples 1 and 2 dealt with an optical fiber 211 made from quartz glassand a protective tube 212 made of stainless steel to measure atemperature of the molten steel at approximately 1500° C. However, atemperature at around 1000° C. could be measured by using an opticalfiber made from multi-component glass having a softening point of 1000°C. or below covered with an insulation coating containing carbonparticles as the additive.

Example 4

Example 4 used an optical fiber covered with a metallic tube, which wasfurther covered with a paper tape winding thereon. The structure of thedouble-covered optical fiber was a 50/125 optical fiber 211 comprising aGI fiber of quartz glass coated by polyimide, a protective tube 212which had 1.4 mm of outer diameter and 1 mm of inner diameter and whichwas made of stainless steel to cover the optical fiber 211, a paper tapeas the insulation coating wound around the protective tube 212.

The prepared double-covered optical fiber was immersed into the moltensteel at a temperature of 1400° to 1600° C. to a depth of 200 mm, andthe state of the tip of double-covered optical fiber was observed. Theportion immersed into the molten steel left soot-like charcoal on thesurface of the metallic tube, and approximately 50 mm length at the tipwas melted out. In the case that an optical fiber covered with ametallic tube without using an insulation coating was immersed into themolten steel, nearly all the portion immersed into the molten steel wasmelted out.

From the observation described above, it was confirmed that thedouble-covered optical fiber using an insulation coating consisting ofcellulose performed the measurement of temperature at a sufficientlydeep zone of a molten metal with a reduced amount of melt-loss in everymeasurement cycle.

As described in detail above, since the double-covered optical fiber ofthe present invention employs a metallic protective tube on the opticalfiber and the insulation coating containing particles having a meltingpoint higher than the temperature of target material or an insulationcoating comprising cellulose, the double-covered optical fiber gives areduced amount of melt-loss to decrease the cost of measurement evenwhen it is applied to the molten metal system.

Also since the metallic protective tube has a sufficient strength, thedouble-covered optical fiber of the present invention allows to measurea temperature at a deep zone in the molten metal for determining acorrect temperature.

What is claimed is:
 1. A temperature measuring device comprising:(a) anoptical fiber; (b) a meltable protective tube for covering and forproviding rigidness to the optical fiber; (c) a heat insulation coatingfor covering the protective tube, the optical fiber being covered withthe protective tube and the heat insulation coating to form adouble-covered optical fiber, and a tip of the double-covered opticalfiber being a radiation sensing element; (d) the optical fiber having atemperature at which the optical fiber is consumed higher than atemperature of a molten metal to be measured; (e) the protective tubeand the heat insulation coating having a heat resistant temperaturelower than a temperature of the molten metal to be measured; and (f) aradiation thermometer connected to the double-covered optical fiber. 2.The temperature measuring device of claim 1, wherein said optical fiberis made from a quartz glass.
 3. The temperature measuring device ofclaim 1, wherein said protective tube is a metallic tube having a heatresistant temperature higher than that of the heat insulation coating.4. The temperature measuring device of claim 3, wherein said metallictube is a stainless steel tube.
 5. The temperature measuring device ofclaim 1, wherein said heat insulation coating is a polyethylene coating.6. The temperature measuring device of claim 1, wherein said heatinsulation coating is a glass fiber coating.
 7. Temperature measuringdevice of claim 1, further comprising:transfer means for sending thedouble-covered optical fiber to a surface of the molten metal and forretracting the optical fiber from the surface to wind the double-coveredoptical fiber; and correction means for correcting a light energyobtained at the tip of the double-covered optical fiber using an initiallength of the optical fiber, a transmission loss per unit length of theoptical fiber, and a change of a feed length of the optical fiberdetected by the transfer means.